This invention relates to processing of semiconductor wafers and, more particularly, relates to apparatus for automated transfer of semiconductor wafers between a cassette type wafer holder and a wafer processing chamber.
Ion implantation has become a standard technique for introducing impurities into semiconductor wafers in a controlled and rapid manner. A beam of ions is generated in a source and directed with varying degrees of acceleration toward the semiconductor wafer. Impurities are introduced into the bulk of semiconductor wafers by using the momentum of the ions as a means of imbedding them in the crystalline lattice of the semiconductor material.
One of the major objectives in commercial semiconductor processing is to achieve a high throughput in terms of wafers processed per unit time. Rapid movement of wafers into and out of the processing chamber is one important factor in attaining high throughput. However, such, wafers are highly fragile and easily damaged due to their thinness (of the order of 10 to 20 mils), large diameter (2 to 5 inches) and highly polished faces which can be rendered useless for device fabrication by many kinds of contamination, abrasion or damage. Thus, extraordinary handling precautions are required and manual wafer handling is undesirable.
Batch processing of a large number of wafers is one way to improve throughput. However, such systems are usually large and expensive to accommodate the batches and involve manual loading of wafers. Furthermore, in the event of a malfunction in a batch processing system, a large number of very expensive semiconductor wafers can be damaged or destroyed.
In serial ion implantation systems, one wafer at a time is processed, and the ion beam is electrostatically scanned over the surface of the wafer until the desired impurity dosage is reached. Such systems are usually smaller and less expensive. However, the wafer transfer time is critical to throughput, since it occurs each time a wafer is processed.
To assist in achieving high throughput, automated wafer transfer systems have been developed. These systems typically transfer wafers from a wafer carrier, or cassette, into a wafer processing chamber and then back into the cassette without intervention by an operator. In one type of automated system, the cassette is mounted vertically with the wafers in a horizontal orientation. The wafers are transferred to and from the processing chamber by various combinations of vacuum picks, air tracks, moving belts and gravity. While such systems provide generally satisfactory operation, there is a tendency for particles to drop on the horizontally oriented wafers and cause contamination.
An automated wafer transfer system, in which the wafers are maintained in a vertical orientation in the cassette and during transfer to and from the processing chamber, is disclosed in U.S. Pat. No. 4,311,427, issued Jan. 19, 1982, to Coad et al. In the disclosed system, an elevator blade edgewise lifts a wafer out of a cassette from below and raises it to a vacuum chuck positioned in the door of a processing chamber. The cassette is positioned over the blade by a chain drive actuated by a stepper motor. The vertical wafer handling system reduces particulate contamination and, due to the edgewise handling of the wafers, reduces abrasion and damage thereof. While such prior art systems provide numerous advantages and generally satisfactory operation, they have certain disadvantages. The elevator blade is actuated by an air cylinder which provides a constant vertical velocity. To avoid breakage of the wafer upon contact, and to avoid exceeding the acceleration of gravity during lowering of the wafer, the speed of the vertical blade must be limited. Therefore, the wafer transfer rate is limited. An additional disadvantage relates to the fact that specially made cassettes are required for compatibility with the cassette conveyor mechanism.
It is a general object of the present invention to provide new and improved apparatus for the vertical transfer of wafers between a cassette and a processing station.
It is another object of the present invention to provide apparatus for precisely positioning a commonly used plastic cassette in an automated wafer transfer system.
It is yet another object of the present invention to provide apparatus for rapidly raising and lowering a wafer from a cassette to a processing station.
It is still another object of the present invention to provide an automated wafer transfer system capable of handling a variety of wafer sizes.
It is still a further object of the present invention to provide an automated wafer transfer system having simple, inexpensive drive systems for positioning a cassette and for lifting wafers.